1. Field of the Invention
The present invention relates to an aberration correction unit for correcting astigmatism caused by an optical system of an optical pickup in an information playback system or an information recording system for playing back or recording record information by irradiating an information recording medium such as a CD or a DVD with a light beam.
2. Description of the Related Art
A DVD (digital video disc or digital versatile disc) is becoming pervasive as an optical disc on which large-capacity digital information can be recorded. The DVD is an optical disc having the same diameter (12 cm) as a CD (compact disc) and is capable of recording digital information of moving pictures, computer information, etc., at a recording density of about eight times or more that of the CD. A DVD playback machine for playing back DVDs can also playback CDs. Different types of optical discs including the CD and DVD are played back with one pickup in the DVD playback machine.
As a method of correcting comatic aberration caused by the tilt angle of each optical disc and spherical aberration caused by the substrate thickness difference between play-backed optical discs to play back the optical discs, JP-A-9-128785 discloses a method of placing liquid crystal elements in an optical path of an optical pickup for correcting the comatic aberration and the spherical aberration. In this method, voltage is applied to the liquid crystal, thereby changing the refractive index of the liquid crystal for giving an appropriate phase difference to the luminous flux passing through the liquid crystal. With the optical pickup thus structured, the effects of the comatic aberration caused by the tilt angle of each optical disc and the spherical aberration caused by the substrate thickness difference between discs are canceled.
However, wave front aberration occurring when an optical disc is played back is caused not only by the comatic aberration or spherical aberration, but also by astigmatism. The astigmatism occurs due to accuracy or an assembly error of optical parts, shift or inclination of an optical axis, or the like. Moreover, even optical pickups each having the same optical system differ in astigmatism direction or aberration size. To decrease the astigmatism, the accuracy of the optical parts used with the optical pickup needs to be improved or after assembly, a precise adjustment needs to be made, resulting in an increase in costs.